The present invention relates to a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery, and relates to a technique for preventing the battery from overheating.
Nonaqueous electrolyte secondary batteries (mainly lithium ion secondary battery) are high voltage batteries with high energy density. Therefore, the nonaqueous electrolyte secondary batteries have been used as main power supply of mobile equipment such as mobile telecommunication equipment, mobile electronic equipment and the like. Furthermore, in recent years, in consideration of environmental problems, high power lithium ion secondary batteries which can be formed to be smaller and lighter than known batteries are used as automobile use batteries or backup power supply batteries.
However, overheating of such lithium ion secondary batteries might be possibly caused by electrical factors such as overcharge, environmental factors such as high temperature exposure, and mechanical factors such as damages caused by fall of a heavy load, and the like. As a method for testing whether overheating of a battery is caused by mechanical factors, there is a method in which, assuming a most severe situation besides dropping a heavy load onto the battery and adding vibration to the battery, an internal short-circuit is generated by sticking a nail (φ4.8) in a charged lithium ion secondary battery (under the assumption that an abuse stated in the SBA standard, in which a nail or the like is mistakenly stuck in a battery when the battery is packaged in wooden case). Tests conducted in the above-described severe situations showed the result that excessive heat of a lithium ion secondary battery is possibly caused in such situations.
Possible causes for this are as follows. First, due to an internal short-circuit, a large current flows and overheating of short-circuited part occurs. Because of the overheating, a separator is fused. Then, a positive electrode plate and a negative electrode plate are brought into full surface contact with one another by the fusion of the separator. Accordingly, a large internal short-circuit occurs, resulting in excessive heat of the battery.
To prevent the generation of a larger internal short-circuit, there has been a proposed structure in which a gap between an inner surface of a battery case and an outer surface of an electrode plate group is formed substantially in a whole area (for example, see Japanese Laid-Open Publication No. 10-321260). According to the proposal, because of the gap between the inner surface of the battery case and the outer surface of the electrode plate group, full surface contact of a positive electrode plate and a negative electrode plate can be avoided, and thus the generation of a larger internal short-circuit can be prevented.
However, a battery described in Japanese Laid-Open Publication No. 10-321260 has a structure in which a gap is formed inside of the battery and thus a volume of an electrode plate group is reduced. Accordingly, a capacity per volume of the battery is reduced.